Replace flex furnace duct with rigid - any tips?

[ObsoleteSum5252]

My Flagstaff 28RKWS has Suburban furnace with 4 flex duct runs. Closest run to furnace is above furnace in bathroom, plenty of heat there. Next shortest run is in hallway on landing next to bedroom, this one also has good heat.  Next shortest is in living quarters, about 8 ft of flex duct away, get a little air out here.  Longest run is in rear kitchen area which is over 16 ft from furnace, hardly any air blows out here.  I found a label on the flex duct that says not intended for runs over 16 ft long.  Makes sense since it probably presents a lot of friction to air passing through it.
To correct this I was thinking of cutting into the under belly,  removing the flexible duct running to the 2 longest runs, and replacing that with rigid aluminum duct.
Has anyone else done this and does it improve airflow?
Also, any recommendations for patching the cuts in the underbelly after I'm done with the work?  My fifth wheel has a semi-rigid plastic corrugated bladder underneath.

 

[grashley]

Sounds like a good plan.  By leaving the two close vents with flex duct, that will also help push more heat down further.

Two other possible solutions: 
A fan installed in the heat duct.
Since it seems the bedroom stays warm, use a box fan or similar placed near the bedroom / bathroom to move heat towards the kitchen during the day.

 

[kdbgoat]

There have been cases where the flex duct was kinked from the factory, cutting down the airflow too. The rigid duct will fix any of that too. Please take lots of pics and keep us updated on your project.

 

[ALLOY1]

Hi

The ducting in our trailer was rebuilt using 4" galv. steel and 3" x 10" registers that have adjustable dampers.  It made a big difference to the air flow.

Two - 4" tank heating ducts were added. These can be closed off when there is no risk of the tanks freezing.

We haven't been out in the cold yet but I suspect our propane and 12VDC use will drop as the increased air flow over the heat exchange will make the furnace more efficient.

All the seams including elbows were sealed with liquid seam sealer not tape

If installing metal ducting isn't going to be easy (ours wasn't) 5"-6" flexible ducting is an option.

 

[Hanr31]

IF moving the underbelly out of the way isn't an option and you must cut it. You could use zip ties to close it up, or sew it up with some braid fishing line. Then I would use some expanding foam as a sealant, or some waterproof tape.


However, before you go tearing into it, verify that there isn't an obstruction. Cover all your other vents and see if the air flow increases. If so, you might same a ton of time by installing vents with louvers to restrict airflow to the rooms that need heat the least.